The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with imaging triggered from the cardiac cycle and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find application in other cardiac gated applications, gated imaging triggered by other moving fluids or tissues, and the like.
In magnetic resonance imaging, a substantially uniform main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a patient being imaged within the examination region. Magnetic resonance is excited in the polarized dipoles by B1 fields generated from radio frequency excitation signals throughout the examination region. Specifically, radio frequency pulses tip the dipoles out of alignment with the main magnetic field and cause the macroscopic magnetic moment to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic resonance signal as the magnetic moment transverse to the direction of the main field relaxes. Magnetic field gradients are applied during this process to encode spatial information in the phase and frequency of the resonance signal. Movement of imaged tissue and the flowing of blood or other fluids disrupts the spatial encoding. The radio frequency magnetic resonance signal is received by the radio frequency coil assembly. From the spatial encoding of the received signals, an image representation is reconstructed for display on a human viewable display.
In cardiac imaging, one of the biggest problems is collecting data when the heart is not moving. If image data gathering is distributed over the whole cardiac cycle, the resultant image is reconstructed from all positions of the heart over the cardiac cycle. Therefore, it is desirable to take multiple image data snapshots from a fixed time segment within the cardiac cycle, so that when the snapshots are combined into a complete data set, the reconstructed image appears as if a still shot were taken of the heart. Selective imaging based on the phase of the cardiac cycle is known as cardiac gating. In addition to imaging the heart, cardiac gating is also useful in imaging regions remote from the heart that are affected by blood flow surges. For instance, an image of a region of the brain that includes an artery, an aneurysm, or other structure that changes with the cardiac cycle is gated.
In order to achieve such images, cardiac activity is monitored and data collection is synchronized with some feature of a monitored signal. In one method of cardiac gating, the pulse of the subject is monitored with an optical transducer placed on a finger of the subject. Light transmission through the finger varies with blood flow. A light signal maximum triggers the collection of data. A fixed delay is introduced to center data collection in other parts of the cardiac cycle. In practice, several factors can affect the timing of the cardiac cycle, for instance, comfort of the subject, health of the subject, distance of the imaged region from the heart, and other factors.
Another method of cardiac gating involves triggering from an electro-cardio-graph (ECG) signal. Typically, three or more electrodes are positioned on the chest of the subject to detect the electrical signals from the brain that control the heart. Each cycle of a normal ECG signal has an acute spike that represents the signal directing the left ventricle to contract. Shortly thereafter, the left ventricle contracts. Typically, in ECG triggered MRI, image data is collected commencing at a time point and continuing for a selected time interval.
Several disadvantages arise from using the ECG signal to trigger magnetic resonance imaging. Metal electrodes and lead wires are used to detect the electrical signal. The electrodes and lead wires cause local abnormalities in the magnetic field, and can distort images. Also, the radio frequency and gradient pulses can induce currents that generate electrode heating which can burn the patient. Extreme electrode heating can burn the patient. The ECG signal is subject to weakening and distortion from several sources, notably, surface resistance, physical condition of the subject, quality of pre-amplifiers, and other external sources. These sources weaken or corrupt the ECG signal making it non-representative of the heart activity in some cases. Additionally, the subject may be sick and not have a strong ECG signal. There are some windows of time where the ECG signal cannot be used as an accurate trigger, for instance in the time immediately preceding the main R-wave spike. It is difficult to obtain an ECG signal during data acquisition, when gradient and RF fields are active. Often the ECG signal is blanked at these times. Also, 1.5 and 3 Tesla magnetic fields distort and interact with the electrical activity making it difficult to detect a useful ECG signal. All of these factors may manifest in a changing baseline, distortion of the signal, experimenting with different electrode arrangements, and others.
The present invention contemplates a new and improved cardiac gating method and apparatus which overcomes the above referenced disadvantages and others.